The present invention relates to methods and apparatus for forming ball bonds in a wire used to make electrical contact with small electrical circuits such as semiconductor integrated circuits. More specifically, one form of the present invention relates to a method and apparatus for forming ball bonds in which a molten ball is formed at a tip of an aluminum wire employing arcs which are essentially simultaneously produced between the tip of a wire and a plurality of electrodes symmetrically disposed around the tip.
In most conventional semiconductor devices, a silicon chip or die is electrically connected to a plurality of arms of a lead frame by segments of fine gold wire for ball bonding or segments of aluminum wire for ultrasonic bonding. Typically, ends of each lead frame arm are located adjacent to and arranged generally radially about the edges of the chip. A corresponding plurality of contact pads are formed in a similar, generally radial array around the peripheral edges of the chip. The contact pads provide electrical connections to electrical structures within the chip. Such a conventional structure is described in greater detail below in connection with FIGS. 1 and 2.
In many conventional applications, the segments of gold wire extend between each pad and its corresponding lead frame arm in order to conduct the electrical current therebetween. Normally, a tip of the wire is drawn through a capillary positioned above the pad and melted to form a molten mass having a ball shape. In many instances, however, the ball is not radially symmetric relative to a principal axis of the wire. When the bond is made, a radially asymmetric ball or "golf club" may be formed in which the wire is predisposed against bending in some directions. Such a radially asymmetric ball or golf club is described in greater detail in connection with FIG. 4a and 4b.
In conventional ball bonding techniques, the ball is affixed to the semiconductor pad. The capillary is then moved laterally towards the corresponding lead frame arm, thereby bending the wire in an arching or bridging configuration towards the corresponding arm. In most applications requiring large numbers of leads and, in particular, in conventional dual-in-line device packages, the lead frame arms are positioned in a generally radial array about the periphery of the chip. The arching or bridging of the wire between each pad and its corresponding arm results in wires being bent in different radial directions. In those instances where golf club bonds are formed, a particular wire may be bent in a direction opposite to the predisposed direction. Such bending can create stress in the wire or bond which could cause a fracture, thereby breaking the electrical path between the pad and the arm.
A well-known expedient for obtaining more symmetric ball bonds is to employ gold wire. In forming the ball bond with gold wire, an arc from a single electrode or flame from a jet is directed toward the tip of the gold wire to melt it. Due to the physical properties of the gold (e.g., surface tension, resistance to oxidation), the melted tip forms a relatively uniform spherical shape in most instances. A principal disadvantage lies in the expense of the gold. Additionally, for reasons of economics, mechanical durability, etc., it may be desirable to make the lead frame arms and pads from materials other than gold. However, when diverse metals are used in the current path from chip to lead, galvanic reactions may occur which could cause deterioration in the electrical connections to the chip.
Accordingly, it would be desirable to use an aluminum wire connected between an aluminum lead frame arm and pad since the use of aluminum would be less expensive and the use of the same metal throughout would inhibit galvanic reactions. However, the physical properties of aluminum create further difficulties in the formation of an aluminum ball with conventional bonding apparatus.
In U.S. Pat. No. 4,098,447 to Edson et al, a method and apparatus are disclosed for forming ball bonds in aluminum wires. The patent teaches the formation of a molten ball by bringing the aluminum wire and a single electrode into temporary contact in a shielding atmosphere, and applying a voltage of less than 200 volts to create a spark discharge. In the method disclosed in U.S. Pat. No. 4,323,759 to Edson et al, a ball bond is formed by initiating a spark discharge across a gap between the aluminum wire and a single electrode in a shielding atmosphere. In this method, a relatively high peak current density is employed.
The above-mentioned techniques create a discharge between a single electrode and the aluminum wire. As a result, a preferential or directionally unbalanced force may act on the tip of the wire. With such an apparatus, a radially asymmetric ball bond may be formed, rather than a well-centered, radially symmetric ball bond.
An object of the present invention is to provide a method and apparatus for forming ball bonds which reduce the difficulties associated with the prior art.
A further object of the present invention is to provide a method and apparatus for forming radially symmetric ball bond from aluminum wire or other type of wire using balanced arc forces.
A further object of the present invention is to provide a method and apparatus for forming ball bonds which reduce the creation of stresses within the wire or bond upon bending.
It is an additional object of the present invention to provide a method and apparatus for forming a ball bond which inhibit the formation of golf club bonds.
It is a further object of the present invention to provide a method and apparatus for forming ball bonds which are less expensive and less likely to cause galvanic reactions within the semiconductor device.
These and other objects and features of the present invention will be apparent from this written description and the appended drawings.